Antimony thioantimonate (SbSbS.sub.4) has been prepared by reaction of antimony oxide (dissolved in conc. potassium hydroxide solution) with sodium thioantimonate (Na.sub.3 SbS.sub.4) in an aqueous medium followed by neutralization of the resulting solution with an acid. See, for example, J. P. King and Yayesh Asmerom, "Investigation of Extreme-Pressure and Antiwear Properties of Antimony Thioantimonate", ASLE Transaction, Vol. 24, 4, 497-504 (1981); and U.S. Pat. No. 3,965,016, to Soulen (issued June 22, 1976). The overall reaction can be written as follows: ##STR1## There are severe limitations associated with the above reaction, i.e., (1) co-precipitation of free sulfur (6%) with the final product and (2) generation of hydrogen sulfide during neutralization. Subsequent studies have shown that a side reaction simultaneously occurs during the neutralization step, as follows: EQU 2Na.sub.3 SbS.sub.4 +6H.sup.+ .fwdarw.SbSbS.sub.4 +3H.sub.2 S+S+6Na.sup.+( 2)
Excessive amounts (more than about 1%) of free sulfur in lubricant additives (such as SbSbS.sub.4 and others) are highly undesirable because the free sulfur promotes corrosion of copper-containing metal parts. Because of the added costs in removing free sulfur from the final product and collecting H.sub.2 S, an improved and economic manufacturing process for preparing SbSbS.sub.4 is highly desirable.
This invention involves a modified reaction route with straight-forward processing conditions to prepare SbSbS.sub.4 having a low sulfur content. The new process can produce SbSbS.sub.4 containing free sulfur at the more acceptable level of about 1% or less. Furthermore, generation of H.sub.2 S during neutralization is virtually eliminated. In addition, the new process does not require isolation and purification of the intermediate, Na.sub.3 SbS.sub.4.8H.sub.2 O. A distinct advantage of the modified reaction route is the fact that the second reaction (Equation 4) is simple and easily controlled. ##STR2##
By carefully controlling the reaction temperatures and pH of the reaction medium good yields of SbSbS.sub.4 containing less than 1% free sulfur can be achieved. The examples listed in Tables I, II and III clearly demonstrate the importance and criticality of the reaction conditions in relation to free sulfur generation and the yields of SbSbS.sub.4. The following descriptions for preparing Na.sub.3 SbS.sub.4 solutions and SbSbS.sub.4 from SbF.sub.3 and SbCl.sub.3 are the preferred procedures.